1. Field of the Invention
This invention relates generally to an R-C network, and more particularly, an R-C network that is fabricated on a single surface of a substrate. One version of the invention is a dissipating terminator used to match the characteristic impedance of a transmission line.
2. Description of the Related Art
Transmission lines are used in a diverse array of electronic equipment to accommodate transmission of electrical or electronic signals. These signals may have a diverse set of characteristics, which might, for example, include direct or alternating currents, analog or digitally encoded content, and modulation of any of a diverse variety of types. Regardless of the characteristics of the signal, an ideal transmission line will conduct the signal from source to destination without altering or distorting the signal. Distance is inconsequential to this ideal transmission line, other than delays that might be characteristic of the transmission medium and the distance to be traversed.
At low frequencies and with direct current transmissions, many transmission lines perform as though they are nearly ideal, even over very great distances. Unfortunately, as the frequency of the signal increases, or as the frequency of component signals that act as a composite increases, the characteristics of most common transmission lines decay and signal transmission progressively worsens. This is particularly true when signals reach the radio frequency range or when transmission lines become longer. One common phenomenon associated with high frequency, long distance transmission lines is a loss of the signal's high frequency components and the introduction of extraneously induced interfering high frequency signals. Another common phenomenon is echo or line resonance, where a signal is reflected from one end of the transmission line back to the other. This echo, in the case of analog voice signals, is commonly known as reverberation, which leads to the effect of one sounding like speech is emanating from within a barrel. The auditory reverberation within a barrel generates a sound similar to the sound after an electrical signal echoes within a transmission line. In the case of a digital pulse, the effect will lead to corrupted data, since additional pulses may be received that were not part of the original transmission, and reflected pulses may cancel subsequent pulses.
In a number of electrical and electronic fields, new circuitry is being developed that has ever increasing capability for higher frequencies. The benefits of these higher frequency components is realized in faster computer processing, in the case of data processing, or broader bandwidth transmissions which can carry more voice signals, more television and radio signals and other signals all over the same communications channel. However, as these communications channels utilize ever-increasing frequencies, the limitations of conventional transmission lines are accentuated. In the case of copper transmission lines, radiation from a signal conductor is dependent directly upon the transmission line length and relative proximity of adjacent signal conductors. So, for example, a long signal line adjacent to another long signal line causes trouble even at lower frequencies. The original telephone lines were twisted in a particular way to reduce signal coupling between separate telephone lines. This signal coupling was aptly referred to in the art by the phrase “cross-talk”, since signals from one telephone conversation would cross the lines into a different telephone line, resulting in talking which crossed the wires improperly. Cross-talk, as aforementioned, is dependent in part upon the spacing between adjacent signal lines. One method of reducing cross-talk is to increase spacing between lines. Unfortunately, another objective in the field of electronics is reduction of the size of components and systems. Simply increasing the spacing often results in greater expense, and also slower overall systems operation speeds—defeating the benefits that were otherwise attained by operating at higher frequencies. Another disadvantage of increased spacing comes from signal radiation. When a copper transmission line is made longer, the conductor will radiate and receive more high frequency energy. So, it is desirable to keep transmission lines shorter, not longer as might otherwise be dictated by cross-talk factors.
To prevent echo within a transmission line, it is possible to terminate the line with a device which is referred to in the art as an energy dissipating terminator. The terminator must have an impedance which is designed to match the characteristic impedance of the transmission line as closely as possible over as many frequencies of interest as possible. Transmission lines generally have an impedance which is based upon the inductance of the conductor wire, capacitance with other signal lines and ground planes or grounding shields, and resistance intrinsic in the wire. With an appropriate transmission line, the sum of the individual impedance components is constant and described as the “characteristic impedance.” To match the transmission line characteristic impedance over a wide frequency range, a terminator must also address each of the individual impedance components. The effect of inductance is to increase impedance with increasing frequency, while capacitance decreases impedance with increasing frequency. Intrinsic resistance is independent of frequency.
In the particular field of data processing, transmission lines typically take the form of busses, which are large numbers of parallel transmission lines along which data may be transmitted. For example, an eight bit data bus will contain at least eight signal transmission lines that interconnect various components within the data processing unit. The data bus is actually a transmission line having to accommodate, with today's processor speeds, frequencies which are in the upper radio frequency band approaching microwave frequencies. These high frequency busses are, in particular, very susceptible to inappropriate termination and transmission line echo.
Terminators used for these more specific applications such as the data processor bus serve several purposes. A first purpose is, of course, to reduce echoes on the bus by resistively dissipating any signals transmitted along the bus. This first purpose is found in essentially all terminator applications. A second purpose, more specific to data busses or other similar electronic circuitry, is to function as what is referred to in the art as a “pull-up” or “pull-down” resistor. The terminator resistor is frequently connected directly to either a positive power supply line or positive power supply plane, in which case the termination resistor is a “pull-up” resistor, or the resistor may be connected to either a negative or ground line or plane, in which case the resistor is referred to as a “pull-down” resistor. When no signal is present on the line, the voltage on the transmission line is determined by the connection of the termination resistor to either a power supply line or a ground or common line. Circuit designers can then work from this predetermined bus voltage to design faster, more power-efficient components and circuits.
The structures of ball grid array R-C terminators are disclosed in the Applicant's Assignee's U.S. Pat. Nos. 6,005,777 and 6,194,979, both of which are explicitly incorporated by reference herein. Generally speaking, each of these terminators includes a ceramic substrate such as a substrate formed from alumina oxide. Resistors formed from a film of conductive-yet resistive material are formed on one surface of the substrate. Capacitors are formed on the opposed surface of the substrate. Each capacitor consists of a first electrode, a dielectric layer and a second layer. The electrodes and dielectric layers applied to the substrate by screen printing processes. Typically, plural resistors and capacitors are formed on each terminator-forming substrate. Conductive vias that extend through the substrate and conductive traces that extend over the surfaces of the substrate connect the capacitors and resistors together to form the desired R-C network.
Solder balls are mechanically and electrically connected to the side of the substrate on which the capacitors are formed. If required by the circuit, the solder balls are connected to the second electrodes of the capacitors. Alternatively, the solder balls are connected to conductors that are connected to other components of the R-C circuit.
An advantage of the above-designed terminators is that the solder balls provide the electrical connection between the terminator and the circuit board conductors to which the terminator is mounted. This eliminates the effort and expense associated with having to precisely solder densely packed terminator leads to complementary densely packed contact pads on the printed circuit board. Another benefit of the above-designed terminators is that the solder balls are disposed within the area subtended by the terminator substrate. Consequently, the complementary contact pads on the circuit board to which this type of terminator is mounted are similarly located under the terminator itself. This reduces the amount of surface area one is required to allocate on a printed circuit board in order facilitate the installation of the terminator.
The above-designed terminators are useful in many applications. However, in these terminators, as in other terminators, the resistors dissipate the applied signals by converting them into heat. There is an increasing interest in using these terminators to dissipate relatively high powered signals. Consequently, the resistors forming a terminator would generate more heat. A concern has arisen that this heat would not, in turn, dissipate away from the resistors. If this occurs, the heat would cause the temperature of the resistors and other components forming the terminator to, over time, break down. If such breakdown occurs, the utility of the terminator could be partially, if not wholly, rendered useless.
Moreover, as discussed above, in order to connect the resistors and capacitors together in the above-described terminators, it is necessary to provide vias through the substrate. This involves forming holes in the substrate and filling the holes with conductive material, typically a metal. Having to perform these steps adds to the overall cost of providing the terminators.